Field of the Invention
The present invention relates to an apparatus for polishing semiconductor wafers and, in particular, one in which the polishing pads are linear, that is, the polishing pads have a long linear dimension relative to their width and have a uniform cross-section along this linear dimension. In addition, the wafer holder travels in a straight line parallel to the long linear dimension of the polishing pads.
Description of the Prior Art
Currently, silicon wafers for semiconductor use are polished with machines which use circular polishing pads that are sometimes rotated. Examples of such machines are disclosed in U.S. Pat. Nos. 4,141,180, 4,193,226, 4,680,893, 4,918,870 and 5,123,214. These machines all provide circular polishing pads to which polishing slurry is added as the silicon wafers are pressed against and passed over the pad surface. The wafers are held in carriers which hold one or more wafers. The carriers may rotate the wafers about a central carrier axis which is parallel to the axis of rotation of the table and polishing pad and may even provide an oscillatory motion to the wafers as they pass over the polishing pad. One disadvantage of this type of polishing machine is that the pieces to be is polished repeatedly traverse the same path or series of tracks. As a result the polishing pad surface wears unevenly resulting in a non-level, concave pad surface. This dishing of the pad produces a convex work piece which is unacceptable. A semiconductor wafer has to be ultra-flat, have a precise thickness and have precise parallel surfaces. Another disadvantage of rotary polishing pads is that the speed of the pad relative to the wafer to be polished varies from the center to the circumference of the pad. Thus the surface contact rate and the polishing rate varies from the center to the periphery of the pad. U.S. Pat. No. 5,020,283 shows a means to make the polishing rate more uniformly providing a circular polishing pad with voids which are more numerous at the periphery of the pad. This is a very complex way to try to make the polishing rate more uniform over the surface of the pad. A further disadvantage is that polishing slurry will not spread in an even manner over a circular pad surface no matter where on the surface it is introduced. Thus, polishing action will vary from place to place on the pad surface not only due to the variation in the speed of the pad relative to the wafer, but also due to the uneven distribution of slurry on the pad. Such differences in polishing action are minimized by the use of linear pads and the straight-line traverse of the wafer carrier. Other disadvantages are apparent when the entire surface of a wafer to be polished is simultaneously in contact with the polishing pad. Polishing slurry trapped between the wafer and the polishing pad causes the wafer to skate, sometimes unevenly, over the surface of the pad as it pushes slurry out from between the wafer and the pad. This skating action can cause uneven wear on the wafer even when it may be rotating relative to the pad. Temperature uniformity is also difficult to control over a pad with a large surface area. Elaborate methods to control temperature uniformity are sometimes used, such as the technique shown in U.S. Pat. No. 5,113,622.